1. Field of the Invention
This invention relates to an element having light or optical waveguides and a method of making the same.
2. Description of the Prior Art
Studies for applying a thin film type optical element, i.e., an optical element using a light waveguide, to a light deflector, a light modulator, a spectrum analyzer, a correlater, an optical switch or the like have heretofore been carried out actively. In such an element having a light waveguide, the refractive index of the light waveguide is varied (by an extraneous action) such as the acousto-optic (AO) effect or the electro-optic (EO) effect, whereby the light propagated through the light waveguide is modulated or deflected. As a substrate forming such an element, use has widely been made, for example, of lithium niobate (hereinafter referred to as LiNbO.sub.3) crystal and lithium tantalate (hereinafter referred to as LiTaO.sub.3) crystal which are excellent in piezo-electric property, acousto-optic effect add electro-optic effect and which have a small light propagation loss.
As a typical method of making a light waveguide by the use of such a crystal substrate, there is a method of heat-diffusing a metal such as titanium (hereinafter referred to as Ti) on the surface of the crystal substrate at a high temperature to thereby form on the surface of the crystal substrate a light waveguide layer having a refractive index slightly greater than the refractive index of the substrate, that is, a Ti internal diffusion method. However, the light waveguide made by this method has a disadvantage that it is susceptible to optical image and can only introduce a light of very small power into the light waveguide. The term "optical damage" refers to "a phenomenon that when the intensity of light input to the light waveguide is increased, the intensity of light propagated through the light waveguide and taken out does not increase in proportion to the intensity of the input light due to scattering."
FIG. 1 of the accompanying drawings is a perspective view show an example of the conventional element having a light waveguide formed by Ti internal diffusion. This example is shown as a case where the element is applied to a high or radio frequency (hereinafter referred to as rf) spectrum analyzer. In FIG. 1, a light emitted from a semiconductor laser 3 is introduced into a light waveguide 2 formed on an LiNbO.sub.3 crystal substrate 1 by Ti internal diffusion and becomes a waveguide light 5 and is collimated by a waveguide lens 6, thus becoming a collimated light 7. On the other hand, a comb type electrode 9 is provided on the light waveguide 2, and an rf power input 8 to be analyzed is applied thereto and a surface acoustic wave (SAW) 10 corresponding to the frequency of this input signal is excited. The aforementioned collimated light 7 is subjected to Brag diffraction by the surface acoustic wave 10 and becomes output lights 12 and 13 Fourier-converted into spectral components by a waveguide lens 11. Thus, in the Fourier conversion surface, a spectrum intensity corresponding to the frequency of said input signal can be observed. The Fourier conversion surface is usually set on the end surface 14 of the light waveguide, and by analyzing the light intensity distribution thereon by means of a photodetector such as CCD, the spectral analysis of the real time of the input signal becomes possible.
In this conventional element, as a method of inputting the waveguide light 5, use is made of the so-called Butt coupling method in which the light-emitting surface 4 of the semiconductor laser 3 is directly brought into contact with the end surface of the light waveguide 2. This Butt coupling method is one of the effective methods o coupling a semiconductor laser to a thin film light waveguide because it can obtain high efficiency and is simple in construction. However, to obtain high efficiency, it is necessary to bring the light-emitting surface 4 of the semiconductor laser into intimate contact with the light waveguide 2, and the input coupling portion becomes remarkably high in power density. Therefore, in the light waveguide made by Ti diffusion as descried above, remarkable optical damage occurs in the input coupling portion, and there has been found a phenomenon that with the loss of light power, scattering of the collimated light 7 by the waveguide lens 6 increases.
Also, as in the aforementioned input coupling portion, in the end surface 14 of the waveguide which provides the Fourier conversion surface, power density becomes remarkably high and thus, optical damage occurs. Thus, it is necessary that the input-output portion of an element having a light waveguide such as an rf spectrum analyzer waveguide a light of high power density therethrough, and it becomes necessary to form a light waveguide having a high resistance to optical damage.
On the other hand, several methods of making a light waveguide have been proposed as methods for overcoming the aforementioned optical damage. Typical ones of such methods are (1) the lithium oxide (hereinafter referred to as Li.sub.2 O) external diffusion method and (2) the ion exchange method. The Li.sub.2 O external diffusion method is a method whereby single crystal such as LiNbO.sub.3 or LiTaO.sub.3 is heat-treated at a high temperature (about 1000.degree. C.) and an Li-lacking layer is formed on the surface of the substrate to thereby form a waveguide. It is known that the light waveguide made by the Li.sub.2 O external diffusion method has a remarkably high resistance to optical damage, as compared with the waveguide made by the Ti internal diffusion (see R. L. Holman: SPIE, Vol. 317, page 47, 1981).
However, it is necessary that the thickness of he light waveguide made by the Li.sub.2 O external diffusion method be made considerably great, e.g., 10-100 .mu.m, because the variation in the refractive index thereof is small. Accordingly, the energy distribution of the waveguide light spreads in the direction of thickness and the interaction thereof with the surface acoustic wave or the like becomes weaker, and this has led to a disadvantage that the efficiency of light modulation or light deflection is remarkably reduced.
The ion exchange method which is another method of making a light waveguide is a method in which an LiNbO.sub.3 or LiTaO.sub.3 substrate is treated in a molten salt containing ions of kalium, silver, etc. The method in which the substrate is treated in a weak acid such as benzoic acid and protons (H) are exchanged as ion product is also used as a method of forming a waveguide. It has been confirmed that the light waveguide made by the ion exchange method has a high resistance to optical damage, as compared with the light waveguide made by the Ti internal diffusion method (see Y. Chen. Appl. Phys. Letl., Vol. 40, page 10, 1982). However, in the ion exchange method, distortion occurs in the crystal during the ion exchange process and, for example, in the element as shown in FIG. 1, the surface acoustic wave attenuates and the interaction thereof with the waveguide light becomes weak, and this has led to a disadvantage that the diffraction efficiency is reduced.